1. Field of the Invention
The present invention relates to a method of performing beam compression of an antenna pattern, which can provide better performances in the beam compression process of an antenna pattern of a radar including a monopulse power feed system.
2. Description of the Related Art
In general, a beam width is one of indexes representing the performance of an antenna pattern of a receiving antenna or other type antennas. A narrower beam width in an antenna pattern will give a better performance. However, there is a relationship of inverse proportion between a beam-width and the size (length) of an antenna. Therefore, if the beam width is reduced, then the size of the antenna will be increased. Conversely, if the dimension of the antenna is reduced, then the beam width will be broadened.
For example, in an antenna for a radar system, if it is desired to double the ability or the resolution to discriminate objects, it is required to halve the beam width and thus it is required to double the size of the antenna. The doubling of the size leads to not only a larger occupied region but also various disadvantages such as an increase in the weight of the antenna and in the size of a structure for supporting the antenna. Conversely, if the size of an antenna is halved, then the beam width will be doubled and the discrimination ability will be degraded by a factor of two.
It is well known that there is such a conflicting relationship between a beam width and the size of an antenna. In most cases, an actual antenna has a limitation in the region it can occupy. Therefore, under these limited conditions, a certain degree of compromise associated with the beam width has to be made.
One known beam compression technique to alleviate the above-described problems is to employ two similar antennas with a monopulse power feed system so as to reduce the beam width by subtracting the difference signal pattern between the two antennas from the sum signal pattern of the two antennas. FIG. 1 is a schematic diagram illustrating a radar system which can perform the beam compression in such a manner described above. In this figure, reference numerals 101,101 designate a pair of antennas with a monopulse power feed system, wherein each antenna has a length of a, and the center-to-center distance between the antennas is d. Reference numeral 102 designates a power divider and reference numeral 103 designates a transmitting circuit. The transmitting circuit 103 generates the transmission power, which is fed in the same phase to the antennas 101 via the power divider 102. Reference numeral 104 designates a hybrid circuit which produces a sum signal .SIGMA. and a difference signal .increment. from receiving signals of the two antennas 101,101. Reference numeral 105 designates a detector for detecting the sum signal .SIGMA. and the difference signal .increment., and reference numeral 106 designates a differential amplifier which provides an antenna output signal by subtracting the difference signal .increment. from the sum signal .SIGMA..
In the radar system having the configuration described above, the differential amplifier 106 provides the antenna output signal produced by subtracting the difference signal .increment. from the sum signal .SIGMA.. One aspect of the antenna output signal is shown in FIG. 2 in a general representing manner of a power pattern associated with beam compression. That is, in FIG. 2, the broken line represents the sum signal .SIGMA. of the receiving signals of the two antennas, and the alternate long and short dash line represents the difference signal .increment.. The solid line represents the output signal (.SIGMA.-.increment.) which is the difference between these two signals. As can be seen, the output signal shows a synthetic directional characteristic having a compressed beam width.
A radar system which performs beam compression according to a conventional technique as described above can provide a certain degree of beam compression. However, if there are a large number of scattering objects in a direction of radio wave radiation, the difference signal .increment. will become small compared to the sum signal .SIGMA.. As a result, the final output signal value obtained by subtracting the difference signal .increment. from the sum signal .SIGMA. will have only a slight difference from the value of the sum signal .SIGMA.. This means that there is a problem that efficient effects of beam compression cannot be achieved.
To solve the above-described problem, the inventor of the present invention has proposed a method of compressing the beam width of an antenna pattern in U.S. patent application Ser. No. 08/209,901 (Japanese Patent Application No. 5-117575). In this previously proposed patent, the method for compressing the beam width of the antenna pattern of a radar system comprises the steps of: providing a radar antenna system comprising two similar antennas having a monopulse feed system; scanning the antenna system while transmitting radio waves from the two antennas of the antenna system in such a manner that both the radio waves transmitted from the respective antennas are in the same phase; receiving by the antenna system the radio waves which were transmitted from the antenna system and reflected by scattering objects and have finally come back; producing a sum signal and a difference signal of the received signals of the two antennas of the antenna system; and performing a signal processing comprising the steps of subtracting the difference signal from the sum signal and providing the resulting signal as a final antenna output signal; the method being characterized in that the above-described signal processing step further comprises the step of providing the final antenna output signal only if the receiving pattern waveform associated with the sum signal has an upwardly convex form, that is, the double differential coefficient of the receiving pattern waveform associated with the sum signal with respect to the scanning angle is negative and further if the receiving pattern waveform associated with the difference signal has a downwardly convex form, that is, the double differential coefficient of the receiving pattern waveform associated with the difference signal with respect to the scanning angle is positive, whereas providing a zero output signal if the above conditions are not satisfied.
In general, when objects to be observed are distributed discretely as in the case of a radar, the receiving pattern waveform has an upwardly convex form near an angle at which an object exists, whereas it does not have an upwardly convex form near angles at which no object exists.
In the case of a beam compression method for a radar antenna system employing two similar antennas with a monopulse feed system, in which the reduction in the beam width is achieved by means of a process using the sum signal and the difference signal of receiving signals of respective antennas wherein the beam compression process is carried out by subtracting the difference signal pattern from the sum signal pattern, the receiving pattern waveform associated with the sum signal has an upwardly convex form near an angle at which an object exists as represented by the broken line in FIG. 2. In contrast, however, the receiving pattern waveform associated with the difference signal has a downwardly convex form near an angle at which an object exists, and the waveform gradually changes to an upwardly convex form as the angle departs from that at which the object exist, as represented by the alternate long and short dash line in FIG. 2.
In the previous invention, therefore, only if the receiving pattern waveform associated with the sum signal has an upwardly convex form, that is, its double differential coefficient is negative, and further if the receiving pattern waveform associated with the difference signal has a downwardly convex form, that is, its double differential coefficient is positive, the difference signal is subtracted from the sum signal and the result is provided as the output signal, and a zero output signal is provided if the above conditions are not satisfied. Thus, the output signal is provided only in the regions where the receiving pattern waveform associated with the sum signal represented by the broken line in the power pattern of FIG. 2 has an upwardly convex form and the receiving pattern waveform associated with the difference signal represented by the alternate long and short dash line has a downwardly convex form. As a result, the output signal is provided in such a manner as represented by the solid line in FIG. 3, which shows a significant improvement of beam compression effect. In FIG. 3, the broken lines represent the regions where the receiving pattern waveform associated with the sum signal has an upwardly convex form, and the alternate long and short dash lines represent the regions where the receiving pattern waveform associated with the difference signal has a downwardly convex form.
In the above description with respect to the method of compressing the beam width of the antenna pattern according to the previous invention, it has been assumed that there is one scattering object within the range of the radio wave radiation. In practical situations, however, there are always different scattering objects having various sizes at various locations. In some cases, for example, if there is a small scattering object near a large scattering object, the receiving pattern waveform associated with the sum signal has an upwardly convex form near the angle at which the large scattering object exists, while it does not have an upwardly convex form near the angle at which the small scattering object exists. For example, if there are point scattering objects in the directions of -12.degree., -6.degree., 0.degree., 6.degree., and 12.degree. wherein the scattering coefficient of the scattering object existing at -6.degree. is 0.1, and the scattering coefficients of the other scattering objects are 1, then the power pattern of the sum signal is such as that shown in FIG. 4, and the power pattern of the difference signal is such as that shown in FIG. 5. As can be seen from FIG. 4, the power pattern of the sum signal does not have an upwardly convex form near the angle of -6.degree. at which the small scattering object exists.
In such a case, the beam-width compression method according to the previous invention provides a zero output signal near the angle at which the small scattering object exists. This means that a zero output signal is provided even when there actually exist some scattering objects. That is, some necessary output signals are missed.